Anisotropic magneto-resistive (AMR) sensors are used in many devices from compasses to steering and positioning devices. These sensors include resistive bridges to provide accurate measurements of weak magnetic fields. (See, for example, U.S. Pat. No. 5,519,318). However, this type of sensor has an inherent offset bias voltage caused by mismatches within the resistive bridge. One scheme for removing this offset is to provide an offset current back into an amplifier connected to the bridge. This offset current is selected for each individual device and selecting the current value can be time consuming for devices used in systems that are mass produced.
Another scheme to reduce the offset is to correct it in the digital domain. The output of the sensor circuit is digitized with an Analog to Digital converter. For example, using the SET and RESET functions on an AMR sensor, 2 measurements are made and then the offset is determined mathematically as (Vset+Vreset)/2. This equation yields the offset voltage and must be subtracted from each measurement using 3 measurements to complete one reading. Another scheme switches between the Set and Reset controls at a fixed rate and uses an integrator to remove the offset. The integrator frequency must be set far below the toggle frequency to extract the offset value and feed it back to a sensor amplifier. This scheme requires extra components and causes the circuit to be physically larger and operationally slower.
Another scheme to reduce the offset includes a servo mechanism which adds complexity and reduces bandwidth due to roll-off response of the integrator involved. And, there may be a critical ratio of clocking frequency to integrator cut-off frequency due to the fact that the bias voltage shows up as an AC signal to the input of the integrator. If the integrator frequency is too high, the sensor bias appears as an AC voltage on the servo feedback voltage. The integrator is used to filter out the AC bias term. A general rule of thumb for clock to integrator cutoff frequencies is 10 times. This limits the useful bandwidth of the sensor. For a wide bandwidth magnetometer, the clock would have to be at least 10 times higher than the integrator frequency and still be stable for the servo operation. The downside of increasing the clock frequency is the power consumption increases and self heating of the sensor bridge may start to affect the operation of the circuit.
All of these conventional schemes described above work for the reduction of V offset, but cause extra steps to be taken to make these sensors useful. Most of the above schemes require a micro processor to perform the calculations. Some schemes can also reduce the available bandwidth or resolution of the sensor circuit. In the digital subtraction scheme, there is also a loss in dynamic range caused by the subtraction of the Vset and Vreset voltages.
Several known schemes of bias reduction all have various draw backs either reducing the dynamic range, using extra measurements to calculate the bias, or various other problems. The typical equation used to remove bias is:Vo(set)=(Vdrv/2)+Voffset+Vmeas  (Eqn. 1)Vo(reset)=(Vdrv/2)+Voffset−Vmeas  (Eqn. 2)Voffset+(Vdrv/2)=(Vo(set)+Vo(reset))/2  (Eqn. 3)Vmag=Vo(set)−(Voffset+(Vdrv/2))  (Eqn. 4)
The terms Vo(set) and Vo(reset) are descriptions of the AMR bridge outputs. Equation 3 indicates the processing required to obtain the resultant voltage that is proportional to the external field. To obtain this resultant voltage a micro processor must manipulate 3 measurements to produce a valid output from the sensor. The repetition rate that Voffset must be measured at is controlled by several factors in the environment such as temperature and level of the external fields to be measured. The Set/Reset functions improve accuracy of the readings and reduce the noise level of the sensor. In rapidly-changing environments, Voffset must be calculated more often to keep the readings accurate as it is used in the final equation to calculate the output of the magnetometer measured level. Voffset can change at a rate of 2700 ppm per degree C. under normal circumstances. It can be seen in the final equation that the Voffset term directly effects the accuracy of the magnetometer measurement. Accordingly, it is desirable to provide correction transparency and temperature independence to the measurements being performed without loss of resolution or reduction in bandwidth.